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Feb 21, 2019

Phytophthora Root Rot of Soybean

Phytophthora root rot (PRR) is common in soybean fields and causes a wet, soft rot of the seed or seedling tissue.

What to Consider

The disease can affect soybean from the seedling stage to near maturity. Stands may be reduced when the pathogen, Phytophthora sojae, infects plants at the seedling stage and causes seed rot and damping off. Infection of older plants causes wilting and browning of leaves and eventual death (Figure 1). Rapid development can occur when soil is high in moisture or there is a periodic rainfall pattern with continued availability of moisture. Soil temperatures above 60° F, and air temperatures between 70 and 77° F favor the disease.1,2 It is common in soils that are -

low lying,

poorly drained,

compacted, and/or

high in clay content.

The chocolate brown discoloration of the stem starts below the ground and extends up the plant. Roots are highly degraded. Later in the season, infected plants yellow and have wilting leaves that remain attached.3 Close attention to symptom development helps distinguish Phytophthora from other diseases such as sudden death syndrome or effects of saturated soils.

Incidence of PRR has become more common with increased use of no-tillage and reduced tillage residue management systems. There are several specific races of the soilborne fungus P. sojae, and fields with tolerant soybean products may maintain yield potential when Phytophthora is present. Some growing seasons present high disease pressure with conditions favoring Phytophthora. The disease may cause an 8 to 11 percent yield loss depending on spring precipitation.1 Soybeans infected with P. sojae are not curable, and management of PRR depends on preventing infections.3

Management

Genes conferring race-specific resistance to Phytophthora are called Rps genes. In some cases, soybean products have more than one Rps gene (race specific) and good field tolerance (race nonspecific), which provides the highest levels of protection from PRR. In order for these products to become susceptible to PRR, a novel variant of Phytophthora would have to become present and widespread.

Phytophthora outbreaks can occur in wet conditions in June. However, upon a closer look at the weather, field conditions, plant symptoms, and genetics, the symptoms may be caused by flooding, another disease such as sudden death syndrome, or other environmental sources and not Phytophthora.

Determining if you have Phytophthora:

Phytophthora-infected plants are more often found as circular patches in low spots or scattered within fields (Figure 3).

Figure 3. Phytophthora affects patches of soybeans fields.

The stem is chocolate-brown colored from the roots and up passed the soil line through the lower stem.

Infection of older plants causes wilting and browning of leaves with drooping petioles.

Plants will have missing lateral roots and the tap root, if present, will be rotted with dark brown to black discoloration throughout and soft, wet tissues that will collapse easily when pinched. This is different than symptoms of soybeans with flooding injury. The outside cortical tissue of roots can be easily stripped in flooded soybean plants. Rhizobium nodules of flooded soybean plants are usually dead instead of displaying a pink ‘functioning’ color.4 Soybeans infected with P. sojae are not curable, and management of PRR depends on preventing infections.3

Seed treatments can help protect against earlyseason infections of PRR for two to three weeks after planting and reduce the risk of a replant situation. Resistance genes (Rps genes) can help protect against specific races of Phytophthora throughout the entire season. Field tolerance is an important tool when there are multiple races of P. sojae in a field. However, field tolerance does not become highly active until plants are approximately at the V1 to V3 growth stage.

In summary, fields should be scouted for PRR and other seedling diseases. Correct identification of this disease is key to management as there are several specific races of P. sojae, and soybean products are identified has having Rps genes that offer protection from this disease.

Sources

1Dorrance, A.E. and Mills, D. 2009. Phytophthora damping off and root rot of soybean. AC-17-09. The Ohio State University Extension. http://ohioline.osu.edu

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